In the related art cellular wireless communication system, in order to utilize a limited frequency resource effectively, an identical frequency resource is used in spatially separated two regions. A frequency reuse rate, which is the subject of a cellular wireless communication system, corresponds to a period of cells where an identical frequency is reused. In a multiple cell communication system where a plurality of cells use a frequency band by division, the frequency band is divided into a plurality of sub-frequency bands whose number is the same as a frequency reuse coefficient (K) to reuse a frequency resource with reduction of interference between cells.
FIG. 1 is a view illustrating a frequency resource allocation in a multiple cell system having a frequency reuse coefficient (K) of 3 (K=3) according to the related art. As shown in FIG. 1, the whole frequency band of the system having a frequency reuse coefficient (K) of 3 is divided into three sub-frequency bands and each cell uses one sub-frequency band.
Since an identical frequency band is not used in the adjacent cells, the adjacent cells do not have an interference between each other. In addition, since the cells using an identical frequency band are separated far from each other, an interference between the separated cells is sufficiently reduced. However, since a ratio of a utilization band to a whole frequency band is reduced to 1/K, a utilization efficiency for a frequency band is also reduced. Accordingly, as the frequency reuse coefficient K increases, the interference between cells is reduced. However, an amount of a frequency resource utilized in each cell is reduced and a whole communication capacity of the system is also reduced. In addition, a ratio of a sub-frequency band utilized in each cell to the whole frequency band, i.e., 1/K may be defined as the frequency reuse rate.
For example, in a code division multiple access (CDMA) system, since each cell uses an identical frequency band, the frequency reuse rate is 1. The frequency reuse rate of 1 means that the whole frequency band is utilized in every cell as the identical sub-frequency band.
An orthogonal frequency division multiple access (OFDMA) system has been suggested as a next generation technology for high speed communication. Similarly to the CDMA system, the OFDMA system is being developed to have a frequency reuse rate of 1. In the CDMA system, an interference between users is minimized by allocating orthogonal codes to the users. In the OFDMA system where a frequency resource is utilized by division both in frequency domain and time domain, although the orthogonality between users in each cell is guaranteed, adjacent two cells has a severe interference because the adjacent two cells use an identical frequency band in an identical time zone. Accordingly, a method of minimizing the interference between the adjacent cells simultaneously with improving an efficiency of frequency resource by obtaining a frequency reuse rate close to 1 has been researched and developed in the OFDMA system.
FIG. 2 is a view illustrating a frequency resource allocation in a fractional frequency reuse (FFR) system suggested by IEEE 802.20 Mobile Broadband Wireless Access (MBWA) standard as a solution for interference at a cell boundary. As shown in FIG. 2, a whole frequency band is divided into a plurality of frequency band and a predetermined frequency band is not utilized in each cell. Accordingly, the adjacent cells use different frequency bands to reduce the interference.
In the FFR system, however, allocation of the plurality of frequency bands should be designed based on a cell position. In addition, when an amount of allocable frequency bands into a position using the predetermined frequency is greater or smaller than the number of users, the frequency resource can not be flexibly allocated. Further, unutilization of the predetermined frequency causes waste in the frequency resource and efficiency of the frequency resource is reduced due to the frequency reuse rate lower than 1.
FIG. 3 is a view illustrating a frequency resource allocation in a soft frequency reuse (SFR) system suggested by 3GPPLTE (3rd Generation Partnership Project Long Term Evolution). As shown in FIG. 3, each cell is divided into a center region and a boundary region. The whole frequency band is utilized in the center region of each cell, while a predetermined frequency band is not utilized in the boundary region of each cell similarly to in the FFR system. As a result, the adjacent cells use different frequency bands. In addition, the communication is performed with a lower power in the center region having an excellent channel state and with a higher power in the boundary region of each cell having a different frequency band from the boundary region of the adjacent cell. Accordingly, users at the boundary region use an improved adaptive modulation and coding (AMC) method.
Similarly to the FFR system, however, the SFR system has disadvantages. Although the center region has a frequency reuse rate of 1, the boundary region does not have a frequency reuse rate of 1 and does not use the predetermined frequency band. As a result, efficiency of the frequency resource is reduced. In addition, when users are distributed to be concentrated at a partial region of the cell, efficiency of the frequency resource is reduce because only the frequency band allocated to the partial region is utilized by users in the partial region. On the contrary, absence of a user in a partial region causes waste in the frequency resource due to unutilization of residual allocable frequency band.
In the FFR system or the SFR system, accordingly, when users are not uniformly distributed, the flexibility in allocation of the frequency resource and the frequency reuse rate of 1 are not obtainable, thereby reducing the efficiency of the frequency resource. Not only the FFR system and the SFR system but also most of frequency resource allocation systems for relieving same channel interference commonly have these problems.